This invention relates generally to imaging technology, and in particular to large-scale imaging systems for modular cargo containers. Specifically, the invention concerns a fast tomography system utilizing a dual neutron-gamma ray beam, and adaptable for use with a large-scale, cost-effective cargo container security program.
Increased global trade has provided substantial economic benefits to a number of world markets, but the commensurate growth in international shipping has raised significant security concerns as well. In particular, modular containers (also known as ISO containers, in reference to the International Organization for Standardization) facilitate cost-effective commerce in a wide range of products, but they also create a vulnerable intermodal shipping conduit through which contraband, weapons, and other dangerous materials can be readily transported, while remaining concealed from existing security systems.
The risk associated with special nuclear materials (SNM) is particularly acute. Special nuclear materials are fissile heavy metals including uranium (U-233 and U-235) and plutonium (particularly Pu-239, but also Pu-238 and other isotopes). Special nuclear materials can be employed in sub-critical reactions (“dirty bombs”), or, with sufficient expertise, used to create crude atomic weapons. In the worst-case scenario, a sufficient quantity of SNM could also serve as the trigger for a high-yield thermonuclear device.
The essential problem is that the quantity of SNM required to pose a strategic risk is quite small, particularly on the scale of a typical ISO container. Specifically, a type I or “strategic” mass of SNM is defined by formula quantity Mf in excess of five kilograms (5 kg), where the formula quantity isMf=mU-235+2.5×(mU-233+mPu)  [1]Variables mU-233, mU-235, and mPu are the masses of uranium-233, uranium-235 and plutonium, respectively, with a two-point-five multiplier on the latter two. Thus a strategic quantity of U-235 is only five kilograms (5 kg), or about the size of a grapefruit. For U-233 and plutonium, only 2 kg is required.
A standard ISO container is eight feet wide (2.44 m), nine feet high (2.59 m) and twenty to forty feet long (6.10 m-12.20 m), with a capacity of twenty tons or more (21,600 kg-26,500 kg). This provides ample volume to conceal strategic quantities of SNM, and to shield them from standard inspection programs. To be effective, therefore, new technologies must provide detailed imaging on an extremely large scale, and must also be fast, efficient and cost-effective enough to handle intermodal traffic measured in the millions of units per month.
Prior art systems have approached this problem via both passive and active detector technologies. Passive detectors search for the characteristic radiation emitted by special nuclear materials, which are radioactive. Because a typical ISO container is so large compared to the type-I mass, however, strategic SNM quantities can be relatively easily shielded, reducing external emissions to a level at which many passive systems become ineffective.
Active systems employ X-ray transmission radiography and other imaging techniques, and are effective at detecting both SNM and associated shielding. Unfortunately, only the highest energy X-rays are sufficiently penetrating for use on a typical ISO container, and at high energy X-rays suffer from low resolution, slow imaging times, and the need for extensive human operator interpretation.
As a result, the majority of ISO container traffic is not subject to effective security screening. There remains, therefore, a need for fast imaging and detection techniques that combine penetrating radiation systems with advance image processing, and are adaptable to a large-scale, cost-effective cargo container security program.